My favorite method of evaluating leaks is to compare the claims against "fan concepts" for lack of a better word. For example, before Smash 4, the universal fan concept of the Duck Hunt dog was a bipedial monstrosity with huge square teeth always prominently visible. When that game's roster leaked, the dog looked quite different and did not match expectations.

Note how little justification there was for the fan concept of a bipedial dog in the first place. It just got carried away for years in the absence of any games and the community fell out of touch with Nintendo's actual thoughts.

This is similar to the wishlist argument expressed in the OP. Only it's not the amount of detail that matters. It's the content of the details.

When I first wrote this comment, I had a prediction: all the fake leaks this year are going to claim cloud saves, just because that's the obvious thing on everyone's mind. Everyone predicts cloud saves. Everyone predicts Netflix. No one even tries to predict Labo. ...Turns out I missed an announcement today, though. This just became an example of how this argument can fail if the "fan concept" is too simple, maybe. But you get the point. Duck Hunt Dog.

I also want to note that default positions are usually bad practice in any kind of debate (a big exception being criminal trials, because consequences). Leaks should not be assumed fake in the absence of any evidence. It doesn't take much evidence to justify throwing a leak away, but you do need something if you're going to take a non-neutral stance. Fortunately, "leaks" almost never stand up to any scrutiny at all.